U.S. patent number 4,906,408 [Application Number 07/274,764] was granted by the patent office on 1990-03-06 for means for the conditioning of radioactive or toxic waste in cement and its production process.
This patent grant is currently assigned to Commissariat a l'Energie Atomique. Invention is credited to Pascal Bouniol.
United States Patent |
4,906,408 |
Bouniol |
March 6, 1990 |
Means for the conditioning of radioactive or toxic waste in cement
and its production process
Abstract
A process for conditioning radioactive or toxic waste which can
contain boron in a cement-based matrix comprises mixing the waste
in a drum in the presence of water with non-aluminous cement,
aluminous cement and optionally a siliceous compound and/or a
boron-containing compound, in order to form a cement matrix
containing stable phases of the straetlingite, calcium
monoboroaluminate and borated ettringite type and, if desired,
placing around the drum a mortar layer, which can be prepared from
cement, deflocculated fumed silica, siliceous sand, smectic clay
and water.
Inventors: |
Bouniol; Pascal (Vincennes,
FR) |
Assignee: |
Commissariat a l'Energie
Atomique (Paris, FR)
|
Family
ID: |
9357387 |
Appl.
No.: |
07/274,764 |
Filed: |
November 22, 1988 |
Foreign Application Priority Data
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|
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Dec 2, 1987 [FR] |
|
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87 16716 |
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Current U.S.
Class: |
588/4; 106/695;
588/11; 588/14; 588/9; 405/129.3; 976/DIG.394 |
Current CPC
Class: |
G21F
9/34 (20130101); G21F 9/165 (20130101); G21F
9/304 (20130101); C04B 28/06 (20130101); C04B
28/02 (20130101); C04B 28/06 (20130101); C04B
7/02 (20130101); C04B 14/04 (20130101); C04B
18/0481 (20130101); C04B 22/0013 (20130101); C04B
24/22 (20130101); C04B 40/0028 (20130101); C04B
41/5076 (20130101); C04B 28/02 (20130101); C04B
14/06 (20130101); C04B 14/104 (20130101); C04B
18/146 (20130101); C04B 2111/00862 (20130101); C04B
2111/00784 (20130101); C04B 2111/00612 (20130101); Y02W
30/94 (20150501); Y02W 30/91 (20150501) |
Current International
Class: |
C04B
28/02 (20060101); C04B 28/06 (20060101); C04B
28/00 (20060101); G21F 9/16 (20060101); G21F
9/30 (20060101); G21F 9/34 (20060101); G21F
009/16 () |
Field of
Search: |
;252/628,633
;250/506.1,507.1 ;106/100,104,85,103 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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2402632 |
|
Apr 1979 |
|
FR |
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2432752 |
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Feb 1980 |
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FR |
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Other References
Chemical Abstracts, vol. 104, 1986, No. 155307v. .
Zement-Kalk-Gips, vol. 34, No. 9, 1981..
|
Primary Examiner: Locker; Howard J.
Attorney, Agent or Firm: Nutter, McClennen & Fish
Claims
I claim:
1. A process for the conditioning of radioactive or toxic waste
which can contain borate ions in a cement-based solid matrix with a
view to the storage thereof, which comprises the following
successive steps:
(a) mixing the waste, in the presence of water, with aluminous
cement, non-aluminous cement and optionally a compound containing
either or both silicon and boron in proportions such that a mixture
is formed, which gives rise to the crystallization of at least one
stable mineral phase selected from the group consisting of the type
straetlingite, calcium monoboraluminate and borated ettringite
phases, apart from the standard hydrates of the non-aluminous
cement, and
(b) allowing the mixture to harden in order to form the solid
matrix containing at least one of these phases.
2. Process according to claim 1, wherein a mortar layer is then
placed around the hardened mixture.
3. Process according to claim 1 wherein the radioactive waste to be
conditioned is constituted by borated liquid concentrates and in
that they are mixed in stage (a) with aluminous cement,
non-aluminous cement based on Portland clinker and a siliceous
compound.
4. Process according to claim 1 wherein in stage (a), the waste is
mixed with water, aluminous cement, non-aluminous cement based on
Portland clinker, a siliceous compound and a compound containing
boron.
5. Process according to claim 1 wherein in stage (a), the
proportions are such that the weight ratio of water/(non-aluminous
cement+aluminous cement+siliceous compound) is below 0.5.
6. Process according to claim 5, wherein the weight ratio of
non-aluminous cement to aluminous cement is in the range 1 to
20.
7. A process according to claim 1 wherein the non-aluminous cement
is selected from the group consisting of ordinary Portland cement
(CPA), Portland cement with an additive (CPJ) and cement with blast
furnace slag and fly ash (CLC).
8. Process according to any one of the claims 1 to 7, characterized
in that the siliceous compound is chosen from the group consisting
of pozzuolanas, clays, metakaolin, kieselguhr, fumed silica, ground
quartz, silica gels, powders of glass and sodium silicate.
9. Process according to claim 1 wherein the boron compound is boric
acid, a borate or borax.
10. Process according to claim 1 wherein in stage (a) a cement
fluidizing agent is added to the mixture.
11. Process according to claim 10, wherein the fluidizing agent is
a naphthalene sulphonate or a melamine-type compound.
12. Process according to claim 2, wherein the mortar layer is
formed from a mixture comprising cement, siliceous sand, fumed
silica, a smectic clay and water.
13. Process according to claim 12, wherein the smectic clay is a
bentonite.
14. Process according to either of claims 12 and 13, wherein the
mixture further comprises a fluidizing agent.
15. Process according to any one of the claims 1, to 4 and 10,
wherein in stage (a), the waste is firstly mixed with the water and
optionally the fluidizing agent, the siliceous compound, part of
the non-aluminous cement and/or the boron-containing compound,
followed by the addition of the aluminous cement and the
non-aluminous cement.
16. Process according to any one of the claims 1 to 4 and 10,
wherein in stage (a), the ratio of the real volume of the waste to
the total volume (waste+water+fluidizing agents+siliceous
compound+boron-containing compound+aluminous cement+Portland
cement) is at the most 0.6.
Description
The present invention relates to the processing, with a view to
long term storage, of radioactive or toxic waste possibly
containing borate ions. More precisely, it relates to the
conditioning of such waste materials in hydraulic binder-based
matrixes constituted by cements.
In nuclear power stations, impurities appearing in the water of the
primary circuit are often eliminated by ion exchange resins in
mixed, i.e. anionic and cationic beds. At the end of the operation,
the used or spent resins essentially contain activation products,
including .sup.60 Co, traces of fission products, including
.sup.137 Cs and a boron-lithium mixture used as the neutron poison
in the water of the circuit, in the form of a H.sub.3 BO.sub.3
-LiOH neutralizer. These spent ion exchange resins constitute
medium activity waste (10.sup.2 to 10.sup.3 Ci/m.sup.3) in view of
their .sup.60 Co content and the period of the latter (T=5.24
years), which must be coated or enveloped with a view to their
storage until the activity disappears.
Among the presently known processing processes, interest is
attached to those involving the incorporation of the waste into
cement due to their relatively low cost and their ease of
realization. However, in the case of ion exchange resins containing
borate ions, it is difficult to use this process due to undesirable
interactions between the resins and the cement mixture or
dimensional variations of the resins, which cast doubts on the
integrity of the coated or enveloped objects obtained.
In nuclear installations, the waste to be treated includes borated
concentrates, which are also medium activity waste materials, whose
incorporation into the cement also causes certain problems.
In order that a process for the conditioning of waste by
incorporation in cement is satisfactory, it is necessary to obtain:
a physical immobilization of the waste in a non-dispersable form,
i.e. a locking in the cement matrix, a retention and confinement of
the radionuclides, in particular the more unstable radionuclides,
such as .sup.137 Cs, a prevention of any water leaching phenomenon,
an adequate mechanical strength of the product obtained to permit
handling and resistance to shocks with a value of at least 12 MPa
after 28 days in accordance with ANDRA specifications, a
dimensional stability of the product obtained, i.e. an absence of
significant contraction or expansion, a correct durability of the
cement matrix with respect to chemical attacks and its own
evolution in time, and an adequate biological protection to not
exceed dose rates of 200 mrads/h in contact with the final
package.
Moreover, it is desirable that the process makes it possible to
incorporate into the cement matrix a waste quantity representing at
least 50% of the apparent volume of the enveloped object obtained
and that the enveloping operations are easy and fast not leading to
the obtaining of new effluents or any other contamination.
When it is wished to incorporate into the cement waste constituted
by ion exchange resins containing borate ions, it is impossible to
obtain the aforementioned results as a result of the prejudicial
effects occurring during the enveloping of the waste and/or during
the ageing of the enveloped product, which are, in part, due to the
affinity of the ion exchange resins for water and, on the other
hand, to the presence of borate ions.
Firstly, ion exchange resins chemically react with the constituents
of the cement and liberate borate ions, which inhibit the setting
of the latter. Thus, on incorporating the resins, into a cement
paste, the mixing water rapidly acquires a very high ionic strength
due to the solubilization of the calcium silicates with the
appearance of Ca(OH).sub.2 and which is adequate to displace part
of the Li.sup.+ and B(OH)-.sub.4 ions, respectively, carried by the
cationic and anionic resins. There follows the dissolving of borate
ions and the precipitation of a not very soluble mineral cortex
based on boron and calcium around the cement grains, which
consequently stops the evolution of their hydration. The
retaining-sealing effect conventionally observed with boric acid
and borates prevents the setting of the enveloped object for
several days or even weeks, thus limiting the production flow of
the conditioned waste.
Moreover, as soon as they come into contact with the cement, the
cationic resins exchange their Li.sup.+ ions for Ca.sup.++ ions,
which are very numerous in the medium and they contract slightly.
This is also applicable with regards to the anionic resins where
the B(OH)-4 ion is partly exchanged with the OH.sup.- ion (size
difference of the hydrated ions).
Secondly, the ion exchange resins have a considerable affinity for
water and their grains can consequently have dimensions which vary
as a function of their water content.
Thus, the behaviour of the cement-resin mixture during its mixing
phase varies as a function of the moisture state of the resin
grains. When using a high resin quantity, the mixture becomes too
stiff if the grains are partly dehydrated, because there is
competition with the cement for the mixing water. However, the
mixture becomes too liquid if there is a water excess compared with
the minimum content enabling the resin grains not to contract.
Moreover, the density contrast between the resin grains (1.2) and
cement particles (2.9 to 3.2) then produces segregations.
These chemical reactions and said affinity for water of the resin
grains also leads to certain prejudicial effects during the
hardening and ageing of the hardened product.
Thus, when the product is able to harden, the mechanical strength
values after 28 days are very low for the most interesting resin
incorporation rates, no matter what the cement quantity used. In
the most favorable cases, with matrix compositions supplying only
very high mechanical strengths, it is not possible to incorporate
more than 30% by volume of resin without dropping below 30 MPa,
this mediocre level corresponding by weight to approximately 20% of
wet resins and 15% of dry resins.
The immersion of the hardened product in water leads to its
expansion and cracking and finally crumbling. This destruction is
due to extremely high tensions appearing locally on each resin
grain (up to a few dozen MPa) following reabsorption of water in
their porous system. Two separate phenomena are responsible for
this:
(a) during setting, the resins have given up water to the cement
hydrates during development and have contracted (a part is also
played by the variation of the osmotic conditions),
(b) the resins have exchanged their ions with the ions present in
the cement and have contracted during hardening.
Thus, as the still soft cement paste initially maintains its
contact with the wall of the resin grains, the diameter of the
latter in the hardened material is below the water saturation
diameter prior to enveloping, so that under these conditions
remoistening of the resins in the enveloped object leads to
bursting.
Moreover, the capillary water present in the resin grains is
subject to radiolysis, which increases the tensions within the
hardened product.
Finally, the ion exchange resins have a poor thermal conductivity,
which does not facilitate the heat dissipation during the setting
of the cement.
In order to overcome these disadvantages, a process is known which
consists of subjecting the resins to a pretreatment by an aqueous
solution containing ions of alkaline earth metals and/or metal ions
of valency at least equal to 3, prior to incorporating them into
the cement and as is described in Japanese patent KOKAI 48/28 899.
However, when the waste materials are constituted by ion exchange
resins containing borate ions, this process does not make it
possible to eliminate the disadvantages due to the liberation of
borate ions in the medium. Moreover, it suffers from the
disadvantage of requiring a preliminary pretreatment stage prior to
the incorporation of the waste into the cement.
Thus, none of the presently known processes makes it possible to
simultaneously overcome the disadvantages due to the affinity of
the ion exchange resins for water and the presence of the borate
ions in the waste.
Another problem occurring in connection with the treatment of the
waste materials constituted by ion exchange resins is the
difficulty of knowing whether these resins do or do not contain
boron and in the case that they contain it how much boron is
present.
In addition, it would be desirable to produce a radioactive waste
conditioning matrix usable in the case of various different waste
types and which could therefore receive cationic, anionic,
regenerated or non-regenerated resins with or without boron or
lithium.
The present invention specifically relates to a process for the
conditioning of radioactive or toxic waste optionally incorporating
borate ions, which makes it possible to overcome the disadvantages
described hereinbefore and to bring about the enveloping of
miscellaneous waste materials which do or do not contain boron.
The process according to the invention for the conditioning of
radio active or toxic waste, which may contain borate ions in a
cement-based solid matrix is based on two aspects:
(a) the production of an enveloping matrix associated with the
synthesis of particular minerals (chemical aspect of the
process)
(b) the optional presence of an over-enveloping material, which is
itself based on cement and makes it possible to improve the
confinement for the waste material packages overall (geometrical
aspect of the process).
The process for the production of the enveloping matrix
comprises:
(a) mixing the waste materials in the presence of water with
aluminous cement, non-aluminous cement and optionally a siliceous
product and/or a compound containing boron in proportions such that
a mixture is formed, which gives rise to the crystallization of
stable mineral phases of the straetlingite, calcium
monoboroaluminate and/or borated ettringite types, apart from the
standard hydrates of the non-aluminous cement and
(b) allowing the mixture to harden to form the solid matrix
containing at least one of these phases.
In this process, as a solid matrix containing a stable phase of the
straetlingite, monoboroaluminate and/or borated ettringite types is
formed around the waste materials, it is possible to avoid the
prejudicial chemical interactions between the optionally borated
waste and the cement or water. Moreover, it is possible to bring
about the hardening of the mixture in one day, while avoiding any
physical or chemical pretreatment operation with respect to the
waste.
Generally, the water, non-aluminous cement, aluminous cement and
siliceous compound quantities used are such that the weight ratio
between the water/(non-aluminous cement+aluminous cement+siliceous
compound) is below 0.5.
Preferably, the weight ratio of the non-aluminous cement to the
aluminous cement is in the range 1 to 20.
When the weight ratio between the water/(non-aluminous
cement+aluminous cement+siliceous compound) is low, e.g. below 0.4,
it is preferable to add to the mixture a fluidizing agent in order
to assist the operation of mixing the constituents. The fluidizing
agent used can be chosen from the group of melamines and napthalene
sulphonates.
According to a first embodiment of the process according to the
invention, the aluminous and non-aluminous cements used in stage
(a) are chosen and there is an addition of a siliceous product and,
optionally, a compound containing boron in proportions such that
between them is formed a straetlingite type-alumino-siliceous
crystalline phase.
This phase can be easily realized at ambient temperature and makes
it possible to attain the following advantages:
(1) formation of a stable compound not subject to a mineralogical
conversion phenomenon, as in the case of aluminous cement hydrates
and also preventing the crystallization of the latter,
(2) development of high mechanical strength characteristics as a
result of favorable crystalline faces (hexagonal system),
(3) good resistance to chemical attacks and to thermal action,
because the decomposition temperature of straetlingite is
200.degree. to 250.degree. C.,
(4) limited dimensional variations in air or under water,
(5) good affinity for radioactive elements such as cesium, which
can be incorporated into the crystal lattice of straetlingite.
In this case, the non-aluminous cement is preferably a cement
formed from Portland clinker suitable for the preparation of prompt
mixtures. Examples of such cements are ordinary Portland cement
(CPA), Portland cement with an additive such as calcarous filler, a
pouzzelan, etc. (CPJ), cement with blast furnace slag and fly ash
(CLC) and blast furnace slag cement (CLK).
The other mineral phases formed in variable proportions in the
matrix and accompanying the straetlingite and conventional cement
hydrates contain boron. These are borated ettringites and calcium
monoboroaluminate, whose structure is based on the known ettringite
and monosulphoaluminate in cements:
6CaO.Al.sub.2 O.sub.3.B.sub.2 O.sub.3.42H.sub.2 O: BOROETTRINGITE
1
6Cao.Al.sub.2 O.sub.3.2B.sub.2 O.sub.3.39H.sub.2 O: BOROETTRINGITE
2
8CaO.2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 25H.sub.2 O:
MONOBORALUMINATE
These highly hydrated minerals consequently bring about an
intervention of aluminium and they can be found in a ternary
diagram CaO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 with all the more
conventionally encountered phases.
FIG. 1 shows such a ternary diagram, in which the considered phases
are as follows:
______________________________________ H.sub.3 BO.sub.3 sassolite
(boric acid), Ca(OH).sub.2 portlandite, Ca[B(OH).sub.4
].sub.2.2H.sub.2 O or hexahydroborite, CaO.B.sub.2 O.sub.3.6H.sub.2
O 8CaO.2Al.sub.2 O.sub.3.B.sub.2 O.sub.3.25H.sub.2 O
"monoboroaluminate" (MBA) 6CaO.Al.sub.2 O.sub.3.2B.sub.2
O.sub.3.39H.sub.2 O "boroettringite 2" (BE2) 6CaO.Al.sub.2
O.sub.3.B.sub.2 O.sub.3.42H.sub.2 O "boroettringite 1" (BE1)
6CaO.Al.sub.2 O.sub.3.36H.sub.2 O "calciettringite" 3CaO.Al.sub.2
O.sub.3 tricalcium aluminate CaO.Al.sub.2 O.sub.3 monocalcium
aluminate Al.sub.2 (OH).sub.6 gibbsite
______________________________________
In this diagram, the compositions corresponding to the formation of
a phase of the boroettringite and monoboroaluminate type are
located in the area of the system defined by boroettringites 1 and
2 and the monoboroaluminate.
These highly hydrated minerals constitute very polymorphous
reception structures for the borate ion B(OH).sub.4.sup.-. Their
limited solubility does not prevent the existence of a certain
quantity of boron in solution, whose function is to regulate the
setting of the mixture of aluminous and non-aluminous cements,
including the presence therein of accelerating ions resulting from
a waste such as lithium.
When the radioactive waste materials to be conditioned are borated
liquid concentrates, it is not necessary to add a boron compound
and, in this case, in stage (a) mixing takes place between the
radioactive waste and the aluminous cement, the non-aluminous
cement based on Portland clinker and a siliceous compound.
However, in the case of other radioactive waste materials, such as
borated or non-borated ion exchange resins, in stage (a) mixing
takes place between the waste and the water, the aluminous cement,
the Portland clinker-based non-aluminous cement, a siliceous
compound and a compound containing boron, the latter for regulating
setting.
According to the invention, the conditioning of the radioactive
waste can be further improved by placing a mortar layer around the
hardened mixture. In this case, the arrangement around the hardened
product of a mortar layer used for over-enveloping makes it
possible to improve its mechanical and confinement properties while
incorporating a higher waste quantity.
Thus, the inventive process has numerous advantages, namely: the
absence of pretreatment greatly simplifies the process overall, the
realization of the solid matrix having mineral phases of the
straetlingite, monoboroaluminate and/or ettringite types is very
simple and only uses commercially available cements, the volume of
waste incorporated into the solid matrix is high and can represent
up to 60% by volume absolute of the mixture, the mechanical
strength of the hardened product obtained in stage (a) is adequate
to permit its handling as from 15 days and there is neither
swelling, nor cracking in the presence of water, and the final
package containing the waste enveloped in the matrix and optionally
the over-enveloping mortar layer satisfies the ANDRA storage
conditions with a mechanical strength respectively equal for each
material to 12 and 35 MPa after 28 days.
The invention will be better understood from reading the following
description given in an illustrative and non-limitative manner and
with reference to the attached drawings, wherein show:
FIG. 1, already described, the theoretical ternary diagram of the
system CaO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3.
FIG. 2 a vertical section through a product obtained by the
inventive process.
According to the first embodiment of the inventive process, the
constituents and the proportions of the constituents to be mixed
with the waste are chosen so as to preferably crystallize a mineral
phase of the straetlingite type.
It is a hydrated calcium aluminosilicate corresponding to the
hydration product of gehlenite. Its structural formula reads:
This mineral can be obtained in numerous different ways, e.g. by
reacting lime with metakaolin, or more simply by reacting calcium
silicate hydrates (CSH) of a Portland clinker-based cement and
calcium aluminate hydrates (CAH.sub.10) of an aluminous cement.
The latter case corresponds to "prompt mixtures", which are
Portland cement-based compositions generally containing between 20
and 80% aluminous cement. These mixtures, whose setting time can be
greatly reduced, are conventionally used for producing sealing
joints, sealing and surfacing purposes, because they have
relatively low mechanical strength characteristics.
In these mixtures, the rapid setting is due to a speeding up of the
hydration of the aluminates by the lime of the Portland cement and,
when the lime is consumed, to an activation of the dissolving of
the silicates. In addition, when these mixtures are hardened in the
absence of borate ions, the very high hydration heat is dissipated
in a short space of time and the temperatures reached for high
material masses lead to contraction, cracking and incomplete
hydration of the cement particles. The limited development and
interlacing of the aluminous or siliceous hydrate crystallites are
responsible for the limited resistance or strength. However, when
these mixtures are hardened in the presence of borate ions, the
situation is greatly modified because this element then serves as a
powerful regulator.
Thus, the hydration of the Portland cement is slowed down and
consequently there is a delay to setting, which compensates the
activation of the aluminous cement, whose hydration is itself
inhibited by boron. This has the following beneficial consequences:
a setting time exceeding 6 hours, but less than 24 hours, a
progressive removal of the heat, slow growth and putting into place
of the hydrate crystals which is favorable for obtaining
appropriate strength or resistance characteristics, a start of
setting taking place with an excess of water so that the borated
waste, e.g. ion exchange resin grains, can retain the maximum size
prior to hardening, a development of primary borated ettringite not
leading to expansion, because it takes place within a still plastic
matrix, whereas this mineral, which expands, normally leads to
cracking when it appears following hardening, and a favorable
rheology of the mixture obviating the use of a fluidizer (solely in
the case of cementation of the concentrates).
Moreover, the mineralogical assembly obtained after hardening has
no aluminous hydrates for which conventionally time-based
recrystallization problems occur.
According to the chemical environment of the borate ions in the
waste, there is also a primary or secondary growth of calcium
monoboroaluminate in the hardened enveloped object without any
particular consequence regarding the quality thereof.
On starting with a prompt mixture containing 50% Portland cement
and 50% aluminous cement, the straetlingite appearance conditions
are as follows:
(1) dissolving of Ca.sup.2+ and AlO.sub.2.sup.- ions leading to a
priority hydration of the aluminous cement or the production of
CAH.sub.10 and C.sub.2 AH.sub.8 hydrates,
(2) the hydration of the Portland cement commences when most of the
aluminous cement has been transformed into hydrates, the
straetlingite appearing as a consequence of this and can be
detected after 1 day.
The water content of the mixture conditions the evolution of the
mineralogical associations. Thus, with a low water/cement ratio,
the least water-rich stable cubic hydrate C.sub.3 AH.sub.6 is
formed with the straetlingite, whose crystallization is limited by
the water deficiency.
Moreover, the lack of reactivity of the C.sub.3 AH.sub.6 hydrate
with respect to the siliceous products of the mixture also prevents
straetlingite formation.
With a high water/cement ratio, the calcium aluminate hydrates
CAH.sub.10 and then C.sub.2 AH.sub.8 precede the appearance of
straetlingite (C.sub.2 ASH.sub.8), which normally forms by reaction
with hydrated silicates (CSH) in accordance with the following
reaction diagrams:
Reaction (1) is faster than reaction (2), but in both cases the
kinetics are controlled by the hydration of calcium silicates:
Thus, in each case lime is formed. However, the presence of the
latter, due to an initial or temporary excess, has the effect of
inhibiting straetlingite formation and aiding that of inert
aluminate hydrate C.sub.3 AH.sub.6.
In addition, for crystallizing straetlingite, it is important not
to associate with the aluminous binder either a cement containing
free lime, or a fortiori pure lime. Preference is therefore given
to the use of a Portland clinker-based cement, such as CLC, CLK,
CPJ, or CPA, as the non-aluminous cement.
To assist the appearance of straetlingite, it is also necessary to
add a siliceous compound to the mixture of the aluminous and
non-aluminous cement with water.
Numerous siliceous compounds can be suitable from the time that
they are able to supply silica. Examples of such siliceous
compounds are pozzuolanas, clays, metakaolin, kieselguhr, fumed
silica, ground quartz, silica gels, powders of glass or sodium
silicate.
In the case where the waste to be conditioned is constituted by
liquids formed by borated concentrates, it is not necessary to add
a boron-containing compound to the mixture. However, in the case of
waste not containing boron or containing a difficultly estimatable
quantity, a water-soluble boron compound is added to the
mixture.
Thus, in mixtures of aluminous cement, cement based on Portland
clinker and a siliceous compound chosen so as to obtain the
stoichiometric composition of the straetlingite, this formation can
be regulated by the presence of boron in the hydration liquid.
The action mechanism can be summarized in the following way:
interaction of the [B(OH).sub.4 ] ions with the Ca.sup.2+ ions in
solution as from the start of mixing, depletion of Ca.sup.2+ ions
in solution and sealing action of the interaction products greatly
inhibiting the hydration of the aluminous cement essentially
constituted by monocalcium aluminate CA, the hydration of the
calcium silicates being completely blocked, hydrates of calcium
aluminates CaH.sub.10 and C.sub.2 AH.sub.8 are formed and cannot be
converted into inert C.sub.3 AH.sub.6, calcium boroaluminates
(boroettringite or monoboroaluminate) then appearing in the place
thereof, with the hydration of the aluminous cement completed, the
calcium silicates start to react to form straetlingite C.sub.2
ASH.sub.8, the boroettringite is generally destabilized to the
advantage of the straetlingite and monoboroaluminate.
Thus, in this case, the completely hydrated and solidified matrix
essentially comprises straetlingite and calcium monoboroaluminate
in stable form.
As hereinbefore, the massive supply of lime must be prevented in
the presence of boron because, apart from the risk of incorrect
setting by hexahydroborite crystallization, there is an activation
of the aluminous binder and a complete deregulation of the system
with excessively fast setting. However, the mixing water can
contain lime up to saturation.
According to the invention, preference is given to the use of
water-soluble boron compounds, e.g. orthoboric acid, borax and
borates in general.
According to the invention, it is also possible to use different
types of aluminous cements. However, preference is given to the use
of cements with a high alumina content, e.g. cements of the Secar
type, rather than cements of the Fondu type which contain too many
impurities. Moreover, Secar cements are white and cannot be
confused with Portland-based cements. In the case of the latter, it
is appropriate to use types, where the sulphate content does not
exceed 2.5% by weight otherwise there may be an interference with
the Ca-Al-B system due to setting deficiencies.
This first embodiment of the inventive process can be used for the
treatment of waste of various types and in particular ion exchange
resins or borated liquid concentrates.
In the case of ion exchange resins, the latter can be cationic
resins in the form H.sup.+ or Li.sup.+, anionic resins in the form
OH.sup.- or [B(OH).sub.4 ], or mixed cationic and anionic exchange
resin beds.
In the case of cationic exchange resins in form H.sup.+, the
affinity of the resin for Ca.sup.2+ ions of the cement is high and
leads to the liberation of H.sup.+, which salts out the acidity.
The calcium taken at the expense of the aluminous and non-aluminous
cements is in low concentration in solution and the mixture is
highly regulated.
In the case of cationic exchange resins in form Li.sup.+, the
affinity of the resin for the calcium ions leads to the liberation
of lithium. However, as lithium is an activating element, a normal
setting of the cement can be obtained.
In the case of anionic exchange resins in form OH.sup.-, the
affinity of the resins for the [B(OH).sub.4 ]-ions present in the
mixture and resulting from the boron compound, leads to the
liberation of OH.sup.- ions in the mixture. Thus, the boron, which
constitutes a setting regulating element is partly consumed by the
resin and setting is normal.
In the case of anionic exchange resins in form [B(OH).sub.4
].sup.-, a high regulation of the mixture is obtained by the
addition of the boron compound.
In the case of mixtures of resins in form H.sup.+ and OH.sup.-, the
setting is relatively highly regulated with the fixation of
Ca.sup.++ ions and the formation of water.
In the case of mixtures of ion exchange resins in form Li.sup.+ and
(B(OH).sub.4)-, setting is normal because there is a liberation of
lithium and therefore an activating element, which compensates the
presence of the inhibiting element.
This embodiment of the inventive process can also be used for the
treatment of borated liquid concentrates. In this case, it is a
question of aqueous solutions containing boron, generally in the
form of sodium borate and it is not necessary to add either water
or a boron compound.
Thus, the boron dissolved in the concentrate is sufficient for
completely regulating the prompt mixture. The accelerating action
of the sodium ion does not totally compensate the inhibiting action
of the boron and the calcium borates which may appear initially are
progressively destroyed in the presence of the aluminium ions of
the aluminous cement to the benefit of the calcium
monoboroaluminate, which is abundantly represented at the sides of
the straetlingite. The latter progressively crystallizes as from
the non-aluminous binder and the siliceous supply, conversion into
C.sub.3 AH.sub.6 being blocked by the boron.
The following examples 1 and 2 illustrate this first embodiment of
the inventive process.
EXAMPLE 1: CONDITIONING OF ION EXCHANGE RESINS
In this example, treatment takes place of ion exchange resins,
which have been compressed and decanted/settled under water, which
corresponds to the storage of ion exchange resins in a dosing
container from which the quantity to be treated is discharged
hydraulically.
The standard porosity state associated with the apparent volume
occupied by the compressed-decanted resins can be defined by a
virtual void index, u corresponding to a porosity n of 25%:
##EQU1## This implies an apparent volume equal to:
The resins are conditioned in cement in such a way as to obtain a
volume enveloping level k.sub.v equal to 2/3, defined by the ratio
V.sub.APP /V.sub.T with V.sub.T representing the total volume of
the enveloped object, i.e. V.sub.T =V.sub.REI +V.sub.E +V.sub.C
with V.sub.E representing the water volume, V.sub.C the cement
volume and REI being the ion exchange resin.
The apparent volume of the ion exchange resins comprises the real
volume V.sub.REI and the saturating water volume V.sub.ESAT, the
mixing water volume V.sub.EGA is such that V.sub.E =V.sub.ESAT
+V.sub.EGA. By choosing an enveloping level k=2/3, it follows that
as V.sub.APP =4/3V.sub.REI, the real volume V.sub.REI =V.sub.T /2,
i.e. the real volume of the ion exchange resins occupies half the
total volume of the enveloped object. Moreover, the saturating
water volume is equal to V.sub.APP -V.sub.REI =V.sub.T /6.
On choosing a relatively low void index of the paste, so as to
compensate the possible water excess supplied by the resins, e.g.
an index u.sub.P =V.sub.E /V.sub.C =0.875, with V.sub.C
representing the total volume of aluminous and non-aluminous
cements and the siliceous compound, we obtain V.sub.C
(1+u.sub.P)=V.sub.P, with V.sub.P representing the paste volume,
which is equal to V.sub.T /2 and to V.sub.C +V.sub.E. Thus, we
obtain:
In the case of an apparent volume of 1 liter of compressed-decanted
ion exchange resins to be treated, the volumes involved are
consequently as follows:
V.sub.REI =750 ml, V.sub.ESAT =250 ml, V.sub.EGA =100 ml, V.sub.C
=400 ml, i.e. V.sub.T =1500 ml and V.sub.P =750 ml.
In order to now estimate the respective quantities of the aluminous
cement, non-aluminous cement and siliceous compound to be used, in
order to obtain the virtual composition of the straetlingite, i.e.
Ca.sub.2 Al.sub.2 SiO.sub.7, nH.sub.2 O, it is possible to operate
in the following way.
On designating by Ca/Al/Si the number of atoms of calcium,
aluminium and silicon respectively necessary for the synthesis of 1
mole of straetlingite, X the weight of non-aluminous cement to be
introduced for 1 mole of straetlingite (weights expressed in g), Y
the weight of aluminous cement for 1 mole of straetlingite, z the
weight of siliceous additive for 1 mole of straetlingite and
Ca.sub.x, Ca.sub.y, Ca.sub.z /Al.sub.x, Al.sub.y, Al.sub.z
/Si.sub.x, Si.sub.y, Si.sub.z the number of atoms which can be
effectively liberated by 1 g of each constituent X, Y or Z, the
weight of the various ingredients leading to the straetlingite
composition is given by solving the system: ##EQU2##
By e.g. choosing: for the non-aluminous binder, a CLC 45 with:
Ca.sub.x =8.114.multidot.10.sup.-3 ; Al.sub.x
=2.118.multidot.10.sup.-3 ; Si.sub.x =4.827.multidot.10.sup.-3, the
value of Ca.sub.x takes account of all the calcium of the cement;
for the aluminous binder: SECAR 71 with: Ca.sub.y
=4.457.multidot.10.sup.-3 ; Al.sub.y =10.789.multidot.10.sup.-3 ;
Si.sub.y =0, the value Al.sub.y not taking account of the inert
--Al.sub.2 O.sub.3 fraction; for the siliceous additive: a fume
silica with: Ca.sub.z =0; Al.sub.z =0; Si.sub.z
=15.595.multidot.10.sup.-3.
The solutions are as follows: ##EQU3##
This calculation shows that the proportions of the aluminous and
non-aluminous cements are substantially equivalent, which
corresponds to a typical prompt mixture. Moreover, the siliceous
compound proportion, i.e. the fumed silica to be used represents
less than 10% of the cement and consequently corresponds to the
dosing recommended for this constituent in cements. With regards to
the boron addition, the latter must be compatible with a complete
solubilization in the water of the mixture in order to avoid the
presence of residual boron source in the matrix.
In the case of orthoboric acid H.sub.3 BO.sub.3, the limited
solubility of said acid (approximately 50 g/1 at 20.degree. C.)
leads to the use of an approximate quantity of 15 g for the liquid
volume associated with the conditioning of 1 liter apparent of
compressed-decanted ion exchange resins, i.e. 350 ml in all.
Consequently, in order to respect the determined liquid volume for
mixing, the water volume V.sub.EGA must be corrected as
follows:
in which m.sub.H.sbsb.3.sub.BO.sbsb.3 represents the orthoboric
acid weight used, i.e. 15 g and m.sub.H.sbsb.3.sub.BO.sbsb.3
represents the volume weight of the H.sub.3 BO.sub.3, 1.5.
In addition, the mixing water volume must only be 90 ml.
On retaining for the various constituents the following volume
weights:
CLC 45: m.sub.x =2.90,
SECAR 71: m.sub.y =3.00,
FUMED SILICA: m.sub.z =2.00,
the weights involved for a total volume of "binders" V.sub.c =400
ml are respectively equal to: ##EQU4##
In this example, the water/cement ratio of the mixture is low and
it is consequently indispensable to add a fluidizing agent in order
to obtain a consistency favorable for mixing directly within the
drum. This fluidizing agent can be incorporated in solid, e.g.
powder form into the cement and the siliceous additive, so as to
deflocculate, as from the first moments of hydration, the particles
of fines and in particular those of the siliceous compound.
The fluidizing agent dose used is dependent on the weight of the
non-aluminous cement and the weight of the siliceous compound,
because the aluminous cement is not very sensitive to its action.
When the fluidizing agent is constituted by a product in the
melanine group of the Melment type, the recommended dose is 1 kg
for 100 kg of cement, so that in this example 6.2 g of fluidizing
agent are used.
In addition, the incorporation of this fluidizing agent helps to
temporarily isolate the ion exchange resin grains from their new
chemical environment by means of a surfactant film. Thus, the
latter limits the violent ion exchanges and the risks of
excessively fast setting of the cement.
The complete formulation used for the conditioning of 1 liter of
compressed-decanted ion exchange resins is definitively as
follows:
REI WASTE: 1 l,
WATER: 90 g,
FUMED SILICA: 49 g,
CLC 45: 569 g,
SECAR 71: 538 g,
H.sub.3 BO.sub.3 : 15 g,
MELMENT: 6.2 g.
The total volume is approximately 1.5 l with a water/cement
equivalent ratio of 0.3.
The following procedure is adopted for conditioning purposes.
Into the drum are firstly introduced the compressed-decanted ion
exchange resins, followed by the mixing water, the fluidizing
agent, the fume silica, the boric acid, the aluminous cement and
the non-aluminous cement.
Thus, it is preferable to incorporate the siliceous additive prior
to the two cements in order to permit an effective deflocculation
in the absence of alkaline pH, the grains of resins aiding the
dispersion by attrition. However, it is also possible to
incorporate the two hydraulic binders, i.e. the aluminous cement
and non-aluminous cement in a simultaneous manner, as a
preformulated composite binder. It is also possible to introduce
together the waste, the mixing water, the fluidizing agent, the
fumed silica and the boric acid, followed by the two cements.
Prior to the introduction of the cements, everything is mixed for
at least 3 minutes. This is followed by the introduction of the
aluminous cement and the non-aluminous cement and mixing is
continued for at least 3 minutes, but for no more than 10 minutes.
The cements are introduced in the liquid phase, whilst controlling
the flow rate, in order not to block the mixing system by a massive
discharge.
At the end of mixing, the mixture has still not hardened and
has:
a liquid consistency for 30 minutes,
a thixotropic character so that the mixture must not be exposed to
vibrations, otherwise the constituents would segregate,
a varying setting between 1 and 24 hours,
an absence of sweating for 24 hours.
Once hardened, the enveloped object has the following
characteristics: an absence of cracking under water, mechanical
strength characteristics exceeding 12 MPA after 28 days permitting
the handling thereof, a mineralogical composition evolving well
beyond that after 28 days related to the mechanical strength
characteristics; mainly straetlingite, calcium monoboroaluminate
and in a subsidiary manner borated ettringite.
The evolution of the material in the confined state, in a tight
enclosure, or preferably under water, leads to an improvement to
the mechanical strength characteristics after 90 days, the
crystallization of the straetlingite being more easily completed in
the absence of drying.
EXAMPLE 2: CONDITIONING BORATED CONCENTRATES
In this case, the presence of boron is perfectly identified and as
the waste to be treated is an aqueous solution, it is possible to
eliminate the addition of orthoboric acid, water and the fluidizing
additive.
It is possible to choose an incorporation level equal to 4/7, i.e.
a V.sub.L /V.sub.T ratio of 4/7 with V.sub.L representing the real
volume of the liquid waste. As V.sub.T =V.sub.L +V.sub.C, with
V.sub.C representing the volume of the cements and the siliceous
additive, we obtain:
Thus, the volumes used for conditioning 1 liter of concentrate are
V.sub.L =1000 ml, V.sub.C =750 ml, i.e. a total volume V.sub.T of
1750 ml.
In this case, the incorporation level of close to 57% determines an
equivalent water/cement ratio of approximately 0.45.
If use is made of the same cements and the same siliceous compound
as in example 1, the weights used for a total binder volume of 750
ml are respectively equal to: ##EQU5##
The formulation for enveloping 1 liter of concentrate is
definitively as follows:
BORATED CONCENTRATE WASTE: 1 l,
FUMED SILICA: 92 g,
CLC 45: 1066 g,
SECAR 71: 1010 g.
In order to carry out conditioning, the procedure of example 1 can
be adopted, the constituents being introduced in the following
order:
(1) liquid concentrate,
(2) siliceous additive,
(3) aluminous cement,
(4) non-aluminous cement.
It is possible to mix together the concentrate and fumed silica for
at least 3 minutes prior to the introduction of the cements in a
successive or simultaneous manner. Following cement introduction,
mixing is continued for 3 to 10 minutes. After said mixing, the
fresh mixture has a liquid consistency for 30 minutes following the
start of mixing, slow setting occurring within 24 hours and an
absence of sweating after 24 hours.
After hardening, the mixture has an absence of cracking under
water, a mechanical compressive strength exceeding 20 MPa after 28
days and increasing significantly thereafter and a mineralogical
composition evolving well beyond 28 days and comprising mainly
straetlingite, calcium monoboroaluminate and borated
ettringite.
According to a second embodiment, the inventive process is
performed for forming in a minority proportion mineral phases of
straetlingite, borated ettringite and/or calcium monoboroaluminate
starting with boron-containing waste.
For this purpose use is made of mixtures of aluminous cement and
Portland cement, highly enriched with the latter and optionally
containing a siliceous compound. With a Portland cement/aluminous
cement ratio exceeding 4, e.g. 10 or 20, the hydration of the
Portland cement is activated by the aluminous cement, but under
conditions remote from those of typical prompt mixtures. Therefore,
the regulation of the mixture requires less boron. In this case,
the hardened matrix contains mainly the conventional hydrated
calcium silicates of the Portland cement and in a small proportion
straetlingite, borated ettringites and/or monoboroaluminate.
As in the first embodiment, the regulation of the setting by the
borate ions of the waste, as well as the addition determines a slow
development of the hydration making it possible to progressively
remove the heat.
The presence of a small amount of water necessary for obtaining
high mechanical strength characteristics also involves the use of a
fluidizing agent.
In order to carry out the conditioning in accordance with this
second inventive embodiment, a rigorous operating procedure must be
respected, particularly with regards to the order of introducing
the different constituents. It is therefore important for the
borated or unborated waste accompanied by the systematic boron
addition is introduced into an aqueous medium already containing
Ca.sup.++ ions in solution.
Thus, most of the boron reacts directly in this case with the
calcium, therefore permitting, following the introduction of the
cement, a normal hydration of the latter. Bearing in mind the small
aluminous cement quantity in this second embodiment, the activation
of the Portland-based cement would not be obtainable if the cement
was introduced following the borated waste, thus leading to the
immediate formation of a borated cortex around the cement grains
with blocking of the hydration reaction.
In order that the boron can react with an already
calcium-containing solution, it is sufficient for this purpose to
introduce the cement in two portions, in accordance with the
following operating procedure, which takes place completely within
the mixing drum:
(a) introduction of the mixing water, half the fluidizing agent and
any siliceous additive,
(b) introduction of 1/5 of the non-aluminous cement and mixing for
a minimum of 3 minutes,
(c) introduction of the waste and the boron (case of REI) with
mixing for a minimum of 3 minutes, and
(d) introduction of the remaining 4/5 of the non-aluminous cement,
then the aluminous cement and the last half of the fluidizing
agent.
During these various operations, mixing must not be interrupted so
as to prevent segregation prior to complete mixing.
EXAMPLE 3
A description will now be given of the use of the second embodiment
for the treatment of an Amberlite resin of type IRN-150L in mixed
beds, containing boron and lithium and which has a water content:
W=52.5% and an apparent density: d.sub.a =1042 g/l apparent.
While using a resin void index of U.sub.REI =1/3 and an apparent
volume incorporation level of the compressed-decanted REI with
their immersion water and V.sub.T the total volume of the enveloped
object (REI+matrix), it follows that V.sub.REI =3/8V.sub.T, i.e.
that the real volume of the ion exchange resins occupies 37.5% of
the total volume of the enveloped object.
Thus:
the paste volume: V.sub.P =V.sub.T -V.sub.REI =5/8V.sub.T
the volume of the cements (and siliceous additive): V.sub.C
=V.sub.P /1=U.sub.P =V.sub.T /3 whilst taking up=VE/VC=0.875
total water volume: V.sub.E =V.sub.P -V.sub.C =7/24V.sub.T
saturation water volume: V.sub.ESAT =V.sub.APP -V.sub.REJ =V.sub.T
/8
mixing water volume: V.sub.EGA =V.sub.E -V.sub.ESAT =V.sub.T /6
The volumes used for cementing 1 liter apparent of
compressed-decanted REI are therefore as follows: V.sub.REI =750
ml, V.sub.ESAT =250 ml, V.sub.EGA =333.33 ml, V.sub.C =666.66 ml,
i.e. V.sub.T =2000 ml and V.sub.P =1250 ml.
The composition of the matrix, i.e. the detail of V.sub.C, is on
this occasion given as a function of the optimum percentages
observed for the aluminous cement Y and the fumed silica Z within
the mixture essentially containing a Portland cement X, i.e.
V.sub.X =80% V.sub.C ; V.sub.Y =10% V.sub.C ; V.sub.Z =10%
V.sub.C.
By using a CPA55, a SECAR 71 and fumed silica, whose respective
volume weights are m.sub.x =3.2, m.sub.y =3.0 and m.sub.z =2.0, the
weights involved for a volume of "binders" of 666.66 ml are:
M.sub.X =1706.7 g, M.sub.Y =200.0 g, M.sub.Z =133.3 g.
Moreover, the addition of soluble boron in the form of orthoboric
acid H.sub.3 BO.sub.3 is brought to 5 g only, bearing in mind the
low aluminous cement proportion, so that the water volume for
mixing must be corrected as follows:
The low water/cement ratio of the mixture requires a high
fluidizing agent dosing, because it is based on the proportion of
CPA and fumed silica.
At a rate of 1 kg per 100 kg of cement, the fluidizer weight used,
which is of the powder Melment type, is 18.4 g.
The complete formulation used for the conditioning of 1 l of
compressed-decanted ion exchange resins is definitively as
follows:
REI WASTE: 1 l,
WATER: 330 g,
FUMED SILICA: 135 g,
CPA 55: 1700 g,
SECAR 71: 200 g,
H.sub.3 BO.sub.3 : 5 g,
MELMENT: 18.4 g.
The total volume is approximately 2 l with an equivalent
water/cement ratio of 0.286.
In order to carry out conditioning, the mixing of the constituents
is carried out in a definitive collection drum using a blade, whose
movement is never interrupted between the time of introducing the
first constituent and up to the time of introducing the last.
The mixing water and fumed silica are firstly introduced in their
entirety with half the fluidizer dose. After mixing for 3 minutes,
1/5 of the Portland cement is reduced, followed at the most 3
minutes later by all the waste and the boric acid. Mixing continues
for a further 3 minutes, prior to the introduction of the remaining
4/5 of the Portland cement and the last half of the fluidizer,
which are themselves mixed for 3 minutes. The aluminous cement is
introduced last with a final mixing of 3 minutes.
After a total mixing time of min. 15 minutes, the still unhardened
mixture has a fluid consistency for 30 minutes with a thixotropic
character and a varying setting speed of between 1 and 24 hours
with an absence of sweating.
Once hardened, the enveloped object has a mechanical strength
better than 20 MPa at 28 days, permitting handling, an absence of
cracking under water and a mineralogical composition revealing a
low proportion of straetlingite and/or borated minerals, such as
monoboroaluminate and boroettringite.
In this second embodiment of the invention, as in the first, the
improvement of the confinement can be obtained through the presence
of an over-enveloping, whose putting into place involves the mixing
means used.
Generally, mixing takes place with a mixer having a "lost" blade,
which is immersed in the enveloped object after use and touches the
bottom of the drum and is then disconnected from the motor block.
The length of the blade is such that its end projects at least 5 cm
over the top of the drum and will subsequently be used for gripping
the drum. Furthermore, in order that said handling can take place
under good conditions, it is necessary for the drum content to be
sufficiently hardened and it is generally preferable to wait 15
days to obtain the requisite mechanical strength.
Thus, when the drum leaves the mixing station, it is passed to a
provisional storage area, e.g. constituted by a baryte concrete
tunnel ensuring an adequate biological protection to permit any
work to take place in the remainder of the area.
Following said storage, over-enveloping can take place by placing
round the drum a mortar layer, giving the necessary mechanical
strength and confinement properties. This mortar layer can be
formed from a mixture including cement, siliceous sand, fumed
silica, a smectic clay such as bentonite and water. The composition
of this mixture is preferably that of a compact mortar containing
very little water, generally less than 30% by weight water.
Generally, to this composition is added a fluidizer in order to
facilitate its placing round the drum by pouring or injecting. This
fluidizer must not entrain air, so as not to reduce the mechanical
performance characteristics of the package and will be all the more
effective if the mortar contains, apart from a large amount of
cement, fumed silica and a clay such as bentonite.
The cement used is a slag-based binder, e.g. a cement of type CLC
or CLK. However, it is also possible to use other cements, such as
CPA.
The conjugate action of the very reactive particles of amorphous
silica and clay gives the material a very low permeability and a
high confinement power.
In order to retain high mechanical strength characteristics better
than 60 MPa after 28 days and prevent excessive contraction, it is
appropriate to limit the clay quantity to less than 5% of the
volume of the fines (cements, sand, fumed silica and clay).
For example, it is possible to use the following mortar
composition:
siliceous sand 0-4 mm: 1170 kg/m.sup.3,
cement CLC 45: 650 kg/m.sup.3,
fumed silica: 130 kg/m.sup.3,
smectic clay: 30 kg/m.sup.3,
water+fluidizer: 260 l/m.sup.3.
In order to place said composition round the drum containing the
waste incorporated into the cement, the following procedure can be
adopted. The drum can be placed within a container, which can e.g.
be of concrete, while maintaining it at the desired distance from
the container walls by a lifting system connected to the blade and
by remote manipulation arms. Into the container and around the drum
is then poured the mortar, i.e. the mixture of cement, siliceous
sand, fumed silica, smectic clay and water, which has been prepared
in a mixer.
For example, it is possible to use a mixer with a capacity of 500
l, which is a commercially available apparatus with multiple blades
having respectively circular and epicycloid movements ensuring a
perfect homogenization of the mixture. After mixing, the mixture is
pumped from a return tank and fed into the container by means of a
short, flexible injection pipe. The pump can be of the volumetric
or mechanical type and is able to exert an injection pressure of at
least 1 MPa. Generally, pouring takes place in two stages, in order
to give the mortar, which is initially thixotropic, the time to
acquire a higher shear strength. Firstly, the mortar is poured very
progressively by gravity in order not to trap air or offcenter the
drum with respect to the container and this takes place up to a
mark made beforehand on the container and which corresponds to the
lining of the bottom, while taking account of possible embedding or
penetration of the drum into the mortar layer.
After allowing the mortar to stand for 30 minutes, the blade is
freed from the lifting system and mortar pouring is continued until
level with the container edge. It is possible to add a
supplementary mortar volume in order to fill the upper part of the
container, because the drum content has been compressed during its
putting into place. However, in general a lead disk is placed above
the drum to complete the radiation protection and also ensures the
ballasting of said drum.
FIG. 2 shows the apparatus for the storage of radioactive or toxic
waste obtained through the process of the invention. It can be seen
that this apparatus comprises a drum 1 containing waste 3 in the
form of a dispersion thereof in a mineral matrix having a phase of
the straetlingite, calcium monoboroaluminate and/or borated
ettringite type. If neccessary, drum 1 is surrounded by a mortar
layer 5 placed around the drum and an outer envelope 7, e.g.
constituted by a concrete container surrounding the mortar layer
5.
The structure of this apparatus makes it possible to incorporate a
high volume of waste into drum 1 without the need for any special
fear regarding the intrinsic reduction of the mechanical
performance characteristics, provided that an adequate cement
quantity is used to permit a tensile stressing of the drum 1 via
the lost blade during the placing of drum 1 in container 7.
In said apparatus, the presence of the mortar layer 5 around drum 1
makes it possible to obtain the necessary mechanical properties and
ensure the confinement and retention of the radioactive or toxic
products. Generally layer 5 is at least 10 cm thick.
The mortar composition is as follows:
siliceous sand 0-4 mn: 1170 kg/m.sup.3,
cement CLC 45: 650 kg/m.sup.3,
fumed silica: 130 kg/m.sup.3,
smectic clay: 30 kg/m.sup.3,
water+fluidizer: 260 l/m.sup.3.
The fluidizer used is of the sulphonated naphthalene type and the
physical characteristics of the fresh mortar are as follows:
flow time to the Marsh cone (diameter 12.5 mm): 100 sec,
setting time: 6 h,
density: 2.24.
The mortar is poured in two portions and then allowed to harden,
the characteristics of the product obtained being determined. The
physical characteristics of the hardened mortar are as follows:
contraction after 28 days: <400 .mu.m/m (tight enclosure)
compressive strength after 28 days: >80 MPa,
bending strength after 28 days: >10 MPa.
The contraction and strength characteristics are measured on
prismatic testpieces of dimensions 4.times.4.times.16 cm.
* * * * *